Optical manipulation apparatus

ABSTRACT

A device configured for radiating a focused electromagnetic beam is proposed. Such device comprises: —a first ( 101 ) and a second ( 102 ) part having respectively a second n 2  and third n 3  refractive index and a first W 1  and second W 2 ; —a first contact area ( 100   e   1 ) intended to be between a host medium having a first refractive index n 1  and in which the micro or nanoparticles are intended to be trapped or moved by a focused electromagnetic beam radiated by the device; —a second contact area ( 100   e   2 ) between the first part and the second part; and —a third contact area ( 100   e   3 ) intended to be between the second part and the host medium. The focused electromagnetic beam results from a combination of at least two beams among a first (NJ 1 ), a second (NJ 2 ) and a third (NJ 3 ) jet beams radiated respectively by the first, second and third contact areas when an incoming electromagnetic wave (IEM) illuminates the device. The device is configured for having a direction of propagation of the focused electromagnetic beam tilted in respect of a direction of propagation of the incoming electromagnetic wave.

1. FIELD OF THE DISCLOSURE

The present disclosure relates to the field of optics and photonics, and more specifically to an optical manipulation device that may be used for example for trapping or moving micro or nanoparticles.

The disclosure can be of interest in any field where neutral particles have to be trapped and/or manipulated as for instance in atomic physics, nonlinear physics, biology and medicine, etc.

2. TECHNOLOGICAL BACKGROUND

This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

Photonic condensed optical beams, or photonic “nanojets” generally relate to the generation of a transverse beam width smaller than the diffraction limit and to propagation over several wavelengths without significant divergence. The structured field of nanojets may induce specific optical forces providing the possibility for micro or nanoparticles manipulation along the nanojet EM (for “ElectroMagnetic”) field trajectories.

Since optical manipulation devices (like optical tweezers) are a powerful non-invasive tool in biological and medical application, the study of micro or nanoparticles manipulation based on the nanojet has stimulated researchers' interest. However, the previous works regarding nanojet-induced optical forces focused on trapping particles along an axis of symmetry of the device used to generate the nanojet.

Recently, the asymmetric dielectric system creating a curved photonic jet (“photonic hook”) was introduced to generate the optical forces for moving particles in a curved trajectory as disclosed for instance in the article by A. S. Ang, A. Karabchevsky, I. V. Minin, O. V. Minin, S. V. Sukhov and A. S. Shalin: “‘Photonic Hook’ based optomechanical nanoparticle manipulator”, Scient. Rep., 2018. It was demonstrated that a particle could go around a glass obstacle or be stably trapped near glass and gold obstacles, which allows new applications in optical manipulation of micro or nanoparticles.

There is thus a need for a new type of optical manipulation device providing more flexible nanoparticle manipulation, in particular through the generation of curved jet beams.

3. SUMMARY

A particular aspect of the present disclosure relates to a device configured for radiating a focused electromagnetic beam in a dielectric host medium having a first refractive index n₁, when an incoming electromagnetic wave illuminates a first face of the device, named illumination face. Such a device comprises:

-   -   a first material having a second refractive index n₂ and having         a first width W₁ along a direction of extension of the first         face, named X-axis; and     -   a second material in contact with the first part and having a         third refractive index n₃, having a second width W₂ along the         X-axis,         with n1<n3<n2, and with W₁+W₂ being greater than an equivalent         wavelength λ in the host medium of the incoming electromagnetic         wave. The first and second materials extend along a direction         orthogonal to the illumination face, named Z-axis, from the         illumination face up to the radiating face of each part,         opposite to the illumination face. The first and second         materials having respectively a first height H1 and a second         height H2 along the Z-axis, where |H2−H1|≤λ/4. The device is         configured for having, when the device is in contact with the         dielectric host medium and when the incoming electromagnetic         wave illuminates the illumination face:     -   a first contact area between the dielectric host medium and the         first material, the first contact area radiating a first jet         beam in a near field zone;     -   a second contact area between the first material and the second         material, the second contact area radiating a second jet beam in         the near field zone;     -   a third contact area between the material and the dielectric         host medium, the

The focused electromagnetic beam results from a combination of at least two beams among the first, second and third jet beams. The device is configured for having a direction of propagation of the focused electromagnetic beam being tilted in respect of a direction of propagation of the incoming electromagnetic wave.

Thus, the present disclosure proposes a new and inventive solution for trapping or moving micro or nanoparticles.

More particularly, when the device comprised in the claimed optical manipulation apparatus is illuminated by the incoming electromagnetic wave, the resulting focused electromagnetic beam (or resulting jet beam) is generated in a direction that is tilted in respect of the direction of propagation of the incoming electromagnetic wave. In that case, the electromagnetic field lines of the outgoing focused electromagnetic beam present a curvature allowing the micro or nanoparticles to be trapped or moved, even around or behind objects present in the vicinity of the radiating face of the device from which the focused electromagnetic beam comes out of the device. This allows for a more flexible manipulation of the micro or nanoparticles.

According to one embodiment, the direction of propagation of the focused electromagnetic beam is tilted in respect of a direction of propagation of the incoming electromagnetic wave as a function of at least part of:

-   -   the first n₁, second n₂ and third n₃ refractive indexes;     -   the first W₁ and second W₂ widths; and     -   the first H1 and second H2 heights.

According to one embodiment, the focused electromagnetic beam results from a combination of the first, second and third jet beams.

According to one embodiment, n₃≥√{square root over (n₁n₂)}, W₁=W₂ and H1≥H_(A), with H_(A) a height, along the Z-axis and relative to the illumination face, of the intersection point of the first and second jet beams.

According to one embodiment, n₃<√{square root over (n₁n₂)} and W₂>W₁.

According to one embodiment, n₃<√{square root over (n₁n₂)}, W₂<W₁ and H1≥H_(A), with H_(A) the height, along the Z-axis and relative to the illumination face, of an intersection point of the first and second jet beams.

According to one embodiment, n₃>√{square root over (n₁n₂)}, W₂<W₁ and H1 is targeted to be equal to H_(A)−λ/2, with H_(A) the height, along the Z-axis and relative to the illumination face, of an intersection point of the first and second jet beams.

According to one embodiment, the height H_(A) fulfils

${H_{A} = \frac{W_{1}}{{\tan\;\Theta_{B1}} + {\tan\;\Theta_{B2}}}},\Theta_{B1}$

and Θ_(B2) being respectively radiation tilt angles of the first and second jet beams in respect of the direction of propagation of the incoming electromagnetic wave.

Thus, the device is configured for having the focused electromagnetic beam tilted in respect of the direction of propagation of the incoming electromagnetic wave e.g. when the incoming electromagnetic wave presents a normal incidence relative to the illumination face of the device.

According to one embodiment, Θ_(B1) and Θ_(B2) are targeted to be respectively equal to

${90^{\circ}} - \frac{\Theta_{{TIR}\; 1} + \alpha_{1}}{2}$

and to

${{90^{\circ}} - \frac{\Theta_{{TIR}\; 2} + \alpha_{2}}{2}},$

where angles α₁ and α₂ are respectively the base angles of the first and second contact areas relative to the X-axis, and where Θ_(TIR1) and Θ_(TIR2) are respectively limit angles of refraction associated with the first and third contact areas.

Thus, the device is configured for having the focused electromagnetic beam tilted in respect of the direction of propagation of the incoming electromagnetic wave e.g. when the first and second parts have nonvertical contact areas relative to the illumination face.

According to one embodiment,

$\Theta_{{TIR}\; 1} = {{{\sin^{- 1}\left( \frac{n_{1}}{n_{2}} \right)}\mspace{14mu}{and}\mspace{14mu}\Theta_{{TIR}\; 2}} = {{\sin^{- 1}\left( \frac{n_{3}}{n_{2}} \right)}.}}$

According to one embodiment, the equivalent wavelength in the host medium, λ, of the incoming electromagnetic wave belongs to the visible light spectrum. For instance, the equivalent wavelength in the host medium of the incoming electromagnetic wave belongs to the range going from 390 nm to 700 nm.

According to one embodiment, at least one of the first and second materials belongs to the group comprising: glass, plastic, a polymer material, oxides and nitrides.

Another particular aspect of the present disclosure relates to an optical manipulation system comprising a device as described above (in any of the disclosed embodiments) and an electromagnetic source configured for generating the incoming electromagnetic wave.

Another particular aspect of the present disclosure relates to the use of a device as described above (in any of the disclosed embodiments) or of an optical manipulation system as described above (in any of the disclosed embodiments) for trapping or moving micro or nanoparticles in the dielectric host medium.

4. LIST OF FIGURES

Other features and advantages of embodiments shall appear from the following description, given by way of indicative and non-exhaustive examples and from the appended drawings, of which:

FIG. 1 illustrates the cross-section view, in the XZ-plane, of a device in contact with a dielectric host medium, the device comprising two parts of two different materials according to an embodiment of the present disclosure;

FIG. 2 illustrates the dependence on n₃ of the angles of deviation of the jet beams generated by the device of FIG. 1 when W1=W2;

FIGS. 3a and 3b illustrate the power density distribution along the X-axis for the jet beams generated by the device of FIG. 1 when λ=550 nm, n₁=1, n₂=1.8, W₁=W₂=600 nm, and in the cut-plane Z₀=550 nm with n₃=1.3 (FIG. 3a ); and in the cut-plane Z₀=1000 nm with n₃=1.6 (FIG. 3b );

FIGS. 4a to 4c illustrate the power density distribution in the XZ-plane for the jet beams generated by the device of FIG. 1 when λ=550 nm, n₁=1, n₂=1.8, n₃=1.6, W₁=W₂=600 nm, and when H1=H2=400 nm (FIG. 4a ), H1=H2=600 nm (FIG. 4b ), H1=H2=900 nm (FIG. 4c );

FIG. 4d illustrates the schematic distribution of the main jet beams in FIG. 4 c;

FIGS. 5a and 5b illustrate respectively the X and Z coordinates of the hot spot position of the jet beams generated by the device of FIG. 1 when λ=550 nm, n1=1, n2=1.8, W1=W2=300 nm and H1=H2=H for various values of n3 and various values of H;

FIG. 5c illustrates the power density distribution of the hot spot generated by the device of FIG. 1 when λ=550 nm, n1=1, n2=1.8, W1=W2=300 nm and H1=H2=H for various values of n3 and various values of H;

FIGS. 6a and 6b illustrate the X coordinate of the hot spot position of the jet beams generated by the device of FIG. 1 as a function of the equivalent wavelength in the host medium, λ, of the incoming electromagnetic wave when n1=1, n2=1.8, W1=W2=600 nm and when H1=H2=H, respectively for n3=1.3 (FIG. 6a ) and for n3=1.6 (FIG. 6b );

FIGS. 7a and 7b illustrate respectively the power density distribution in the XZ-plane and the schematic distribution of the jet beams generated by the device of FIG. 1 when λ=550 nm, n₁=1, n₂=1.8, n₃=1.3, W₁=600 nm, W₂=1400 nm and H1=H2=400 nm;

FIG. 7c illustrates the schematic distribution of the main jet beams of FIGS. 7a and 7b in the cut-plane Z₀=1900 nm and for different values of W₂;

FIGS. 8a and 8b illustrate respectively the power density distribution in the XZ-plane and the schematic distribution of the jet beams generated by the device of FIG. 1 when λ=550 nm, n₁=1, n₂=1.8, n₃=1.6, W₁=600 nm, W₂=1400 nm and H1=H2=400 nm;

FIG. 8c illustrates the power density distribution of the hot spots of the jet beams of FIGS. 8a and 8b as a function of W₂;

FIGS. 8d and 8e illustrate respectively the X and Z coordinates of the hot spots of the jet beams of FIGS. 8a and 8b as a function of W₂;

FIGS. 9a and 9b illustrate respectively the power density distribution in the XZ-plane and the schematic distribution of the jet beams generated by the device of FIG. 1 when λ=550 nm, n₁=1, n₂=1.8, n₃=1.6, W₁=1400 nm, W₂=600 nm and H1=H2=400 nm;

FIG. 9c illustrates the schematic distribution of the jet beams generated by the device of FIG. 1 when λ=550 nm, n₁=1, n₂=1.8, n₃=1.6, W₁=1960 nm, W₂=600 nm, H1=H2=400 nm;

FIG. 9d illustrates the power density distribution of the hot spots of the jet beams of FIGS. 9a and 9b as a function of W₁;

FIGS. 10a and 10b illustrate the power density distribution in a XZ-plane of the jet beams generated by the device of FIG. 1 when λ=550 nm, n₁=1, n₂=1.8, W₁=1000 nm, W₂=700 nm, H1=H2=900 nm, and when n₃=1.3 (FIG. 10a ) or n₃=1.6 (FIG. 10b );

FIGS. 11a and 11b illustrate the angle of deviation, relative to the direction of propagation of the incoming electromagnetic wave, of the focused jet beam generated by the device of FIG. 1 when λ=550 nm, n₁=1, n₂=1.8, n₃=1.6, W₁=1000 nm, W₂=700 nm, H1=H2=900 nm, and when α=0° (FIG. 11a ) or α=10° (FIG. 11b ), with α the angle of incidence of the incoming electromagnetic wave relative to the direction normal to the surface of the illuminated face of the device.

FIG. 12 illustrates the cross-section view, in the XZ-plane, of a device comprising two parts of two different materials according to another embodiment of the present disclosure;

FIGS. 13a and 13b illustrate the power density distribution along the X-axis of the jet beams generated by the device of FIG. 12 when λ=550 nm, n₁=1, n₂=1.8, W₁=W₂=600 nm, α₁=85°, α₂=90°, α₃=80°, and in the cut-plane Z₀=550 nm with n₃=1.3 (FIG. 13a ); and in the cut-plane Z₀=1000 nm with n₃=1.6 (FIG. 13b );

FIGS. 14a to 14c illustrate the power density distribution in a XZ-plane of the jet beams generated by the device of FIG. 12 when λ=550 nm, n₁=1, n₂=1.8, n₃=1.6, W₁=W₂=600 nm, α₁=85°, α₂=90°, α₃=80°, and when H1=H2=400 nm (FIG. 14a ), H1=H2=600 nm (FIG. 14b ), or H1=H2=900 nm (FIG. 14c );

FIGS. 15a and 15b illustrate the specific height H_(A) as a function of the base angles α₁, α₂ and α₃ of the device of FIG. 12 when n₁=1, n₂=1.8, n₃=1.6 and W₁=W₂=600 nm;

FIGS. 15c and 15d illustrate the deviation angle Θ₈₆ as a function of the base angles α₁, α₂ and α₃ of the device of FIG. 12 when n₁=1, n₂=1.8, n₃=1.6 and W₁=W₂=600 nm;

FIG. 16 illustrates the power density distribution along the X-axis and in the cut-plane Z₀=1000 nm of the jet beams generated by the device of FIG. 12 for different values of the base angle α₃ when λ=550 nm, n₁=1, n₂=1.8, n₃=1.6, W₁=W₂=600 nm, H1=H2=800 nm, α₁=85°, α₂=90°;

FIGS. 17a and 17b illustrate the optical forces generated by a single part material having a refractive index of n=n1=n2=1.6 (FIG. 17a ), and by an optical manipulation apparatus comprising the device of FIG. 1 when λ=550 nm, n₁=1, n₂=1.8, n₃=1.6, W₁=W₂=600 nm and when H1=H2=900 nm (FIG. 17b ).

FIGS. 18a and 18b illustrate the optical forces generated by a single part material having a refractive index n in presence of an obstacle when n=1.6 and H=900 nm (FIG. 18a ), and when n=1.8 and H=900 nm (FIG. 18b );

FIGS. 18c and 18d illustrate the optical forces generated by an optical manipulation apparatus according to the disclosure and in presence of an obstacle when n₂=1.8, n₃=1.6 and H1=H2=900 nm (FIG. 18c ), and when n₂=1.8, n₃=1.6 and H1=H2=600 nm (FIG. 18d );

FIGS. 19a and 19b illustrate the optical forces generated by a single part material having a refractive index n in presence of an obstacle when n=1.6 and H=900 nm (FIG. 19a ), and when n=1.8 and H=900 nm (FIG. 19b );

FIGS. 19c and 19d illustrate the optical forces generated by an optical manipulation apparatus according to the disclosure and in presence of an obstacle when n₂=1.8, n₃=1.6 and H1=H2=900 nm (FIG. 19c ), and when n₂=1.8, n₃=1.6 and H1=H2=600 nm (FIG. 19d ).

The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

5. DETAILED DESCRIPTION

In all of the figures of the present document, the same numerical reference signs designate similar elements and steps.

The present disclosure relates to a technique for generating optical forces through jet beams which field lines exhibit curvatures allowing micro or nanoparticles to be trapped or moved e.g. around obstacles. This is achieved through the use of a device comprising two or more parts of dielectric materials with different refractive indexes. The refractive indexes of the constitutive parts are higher than the surrounding host medium in which the micro or nanoparticles are trapped or moved. The constitutive parts are configured in such a way that at least two of the jet beams, originating from different contact areas (associated with different parts) of the device, recombine and contribute to the formation of a focused electromagnetic beam which direction of propagation is deflected in respect of a direction of propagation of an incoming electromagnetic wave illuminating the device.

Numerical simulations show that the deflection of a generated focused jet beam can be controlled by the parameters of the device in view of the surrounding host medium.

5.1 Topology

FIG. 1 illustrates the cross-section view, in the XZ-plane, of a device 100 comprising a first part 101 and a second part 102 according to an embodiment of the present disclosure.

The device 100 is configured to be in contact with a dielectric host medium 103 having a first refractive index n₁ and in which the micro or nanoparticles are intended to be trapped or moved by a focused electromagnetic beam radiated by the device 100 when an incoming electromagnetic wave IEM radiated by an electromagnetic source 100 s illuminates at least one face of the device 100, named illumination face 100 i. More particularly:

-   -   the first part 101 is made of a first material having a second         refractive index n₂. The first part 101 has a first width W₁         along the X-axis (the X-axis corresponds in the chosen         coordinate system to a direction of extension of the         illumination face 100 i); and     -   the second part 102 is made of a second material having a third         refractive index n₃, with n₁<n₃<n₂, and having a second width W₂         along the X-axis.

For instance, the first and second materials belong to the group comprising glass, plastic, a polymer material, oxides and nitrides.

The first part 101 and the second part 102 are located side by side along the X-axis with W₁+W₂ greater than the equivalent wavelength in the host medium, λ, of the incoming electromagnetic wave IEM radiated by the electromagnetic source 100 s. The first part 101 and second part 102 extend along the Z-axis (the Z-axis is orthogonal to X-axis and thus to the illumination face 100 i in the chosen coordinate system) from the illumination face 100 i up to another face of the device 100, named radiating face 100 r, opposite to the illumination face 100 i. The first part 101 and second part 102 have respectively a first height H1 and a second height H2 along the Z-axis.

In the embodiment of FIG. 1, H1=H2. However, in other embodiments, the first height H1 and the second height H2 can be different. Indeed, simulations show that the technical effect of having the electromagnetic field lines of the outgoing focused electromagnetic beam presenting a curvature is achieved as long as |H2−H1|≤λ/4. Indeed, as long as this condition is fulfilled, having a difference between the first height H1 and the second height H2 does not lead to the creation of additional jet beams (or secondary lobes) in the focused electromagnetic beam radiated by the device 100.

Back to FIG. 1, the direction of propagation of the incoming electromagnetic wave IEM is orthogonal to the illumination face 100 i (i.e. the direction of propagation of the incoming electromagnetic wave IEM radiated by the electromagnetic source 100 s is parallel to the Z-axis).

However, in other embodiments, the direction of propagation of the incoming electromagnetic wave IEM may be tilted relative to the Z-axis as discussed below in relation with FIGS. 11a and 11 b.

Back to FIG. 1, the first part 101 and the second part 102 have vertical edges parallel to the Z-axis. The radiating 100 r and illumination 100 i faces are orthogonal to the Z-axis, which corresponds to a base angle of 90°.

However, in other embodiments, some prismatic structures (with arbitrary base angles) can also be used for the device 100 as discussed below in relation with FIG. 12. Variation of the base angle value provides an additional degree of freedom in the control of the jet beams radiation.

Back to FIG. 1, the device 100 thus comprises:

-   -   a first contact area 100 e 1 between the host medium 103 and the         first part 101 when the device is in contact with the host         medium 103. The first contact area 100 e 1 contributes to the         radiation of a first jet beam, NJ1, in a near field zone of the         device 1 when the incoming electromagnetic wave IEM illuminates         at least the illumination face 100 i;     -   a second contact area 100 e 2 between the first part 101 and the         second part 102. The second contact area 100 e 2 contributes to         the radiation of a second jet beam, NJ2, in the near field zone         of the device 1 when the incoming electromagnetic wave IEM         illuminates at least the illumination face 100 i; and     -   a third contact area 100 e 3 between the second part 102 and the         host medium 103 when the device 100 is in contact with the host         medium 103. The third contact area 100 e 3 contributes to the         radiation of a third jet beam, NJ3, in the near field zone of         the device 1 when the incoming electromagnetic wave IEM         illuminates at least the illumination face 100 i.

More particularly, when the incoming electromagnetic wave IEM presents an oblique angle of incidence (i.e. the angle of incidence of the incoming electromagnetic wave IEM relative to a normal of the illumination face 100 i), the incoming electromagnetic wave IEM illuminates the illumination face 100 i, but also at least another lateral face of the device 100. Whatever the considered slant angles of incidence, the first contact area 100 e 1, the second contact area 100 e 2 and the third contact area 100 e 3 contribute respectively to the radiation of the first jet beam NJ1, of the second jet beam NJ2 and of the third jet beam NJ3. Such jet beams are obtained from the superposition of a plurality of contributions resulting from the diffraction or refraction of the incoming electromagnetic wave IEM by the different faces or contact areas (e.g. between parts) of the device 100. Thus, in any case the first contact area 100 e 1 contributes to the radiation of the first jet beam Nil, the second contact area 100 e 2 contributes to the radiation of the second jet beam NJ2 and the third contact area 100 e 3 contributes to the radiation of the third jet beam NJ3.

Back to FIG. 1, the three jet beams, NJ1, NJ2 and NJ3 can intersect in different hot spots, where a hot spot corresponds to a point of highest EM power in the considered jet, or focused points, which locations are referenced as points A, B and C.

According to the present disclosure, the materials and size of the first part 101 and of the second part 102 can be optimized in order to manage the positions of the jet hot spots, EM powers, directions and angles of deviation of the three jet beams NJ1, NJ2 and NJ3. As a result, the device 100 behaves as if it was radiating a focused electromagnetic beam resulting from the combination of at least two beams among said first jet beam NJ1, second jet beam NJ2 and third jet beam NJ3. The device 100 can thus be configured for having a direction of propagation of the focused electromagnetic beam being tilted in respect of a direction of propagation of the incoming electromagnetic wave as a function of at least part of:

-   -   the first refractive index n1, the second refractive index n2         and the third refractive index n3;     -   the first width W₁ and the second width W₂; and     -   the first height H1 and the second height H2.

5.2 Design Principles & Main Performance Characteristics

In this Section, a set of equations is provided for estimating example optimal combinations of materials and dimensions of the blocks for having a jet beam shift (i.e. having a tilt angle relative to the direction of propagation of the incoming electromagnetic wave) and deviation. As shown below, the hot spot position and the direction of beam deviation are sensitive to the sizes of constitutive parts. For devices with dimensions larger than a few wavelengths, the Fresnel diffraction phenomenon may have a huge impact.

5.2.1 Main Characteristics of the Generated Jet Beams

Generally, the beam-forming phenomenon appears on a contact area between two materials of different refractive indexes, and is associated with this contact area (e.g. contact area 100 e 1, 100 e 2 or 100 e 3 of the device 100 of FIG. 1) and the illumination face 100 i.

More particularly, the jet beam radiation angle can be derived in relation to the Snell's law. For instance, in the case of the first contact area 100 e 1 of the device 100, when in contact with the dielectric host medium 103, the radiation angle of the first jet beam NJ1 associated with the first contact area 100 e 1 is determined using the approximate formula:

$\begin{matrix} {{\Theta_{B1} \approx \frac{{90^{\circ}} - \Theta_{TIR1}}{2}},} & (1) \end{matrix}$

where

$\Theta_{TIR1} = {\sin^{- 1}\left( \frac{n_{1}}{n_{2}} \right)}$

is the corresponding critical angle of refraction under the assumption that n₂>n₁.

It can be noted that in general, the point of intersection of two jet beams radiated from opposite sides of an element determines the focal length of that element behaving as a microlens. In a first approximation, in the case of a single material element, the focal length of the microlens can be characterized as the function of the width and index ratio of the materials inside and outside the lens. The radiated electromagnetic beam will be directed along the symmetry axis of the element and the focal length of the resulting microlens can be estimated as:

$\begin{matrix} {{F = \frac{W_{1}\gamma}{2}},} & (2) \end{matrix}$

where

$\gamma = \frac{1}{\tan\Theta_{B1}}$

and W₁ is the width of the element.

Back to FIG. 1, the second jet beam NJ2 associated with the second contact area 101 e 2 is refracted at the angle Θ_(B2) into the medium with higher refractive index. Assuming n₂>n₃, the angle Θ_(B2) is obtained as:

$\begin{matrix} {{\Theta_{B2} \approx \frac{{90^{\circ}} - \Theta_{{TIR}\; 2}}{2}},} & (3) \end{matrix}$

where

$\Theta_{{TIR}\; 2} = {\sin^{- 1}\left( \frac{n_{3}}{n_{2}} \right)}$

is the corresponding limit angle of refraction.

Accordingly, the third jet beam NJ3 associated with the third contact area 101 e 3 is refracted at the angle Θ_(B3) with:

$\begin{matrix} {\Theta_{B\; 3} \approx {\frac{{90^{\circ}} - \Theta_{TIR3}}{2}.}} & (4) \end{matrix}$

where

$\Theta_{{TIR}\; 3} = {\sin^{- 1}\left( \frac{n_{3}}{n_{2}} \right)}$

is the corresponding limit angle of refraction.

The length and intensity of the three jet beams NJ1, NJ2 and NJ3, are generally different. The maximal intensity and minimal length correspond to the beam with highest ratio between the refractive indexes, which corresponds to the first jet beam NJ1 refracted at the angle Θ_(B1) when n₂>n₃>n₁.

The behavior of the focused electromagnetic beam radiated by the device 100 may be explained when determining the points of intersection of the three jet beams NJ1, NJ2 and NJ3 radiated respectively at the angles Θ_(B1), Θ_(B2) and Θ_(B3).

The point A of intersection of the first NJ1 and second NJ2 jet beams has the coordinates (W_(A), H_(A)) in the XoZ coordinate system of FIG. 1. More particularly, based on the previous equations,

$\begin{matrix} {{W_{A} \approx {\tan{\Theta_{B2} \cdot H_{A}}}},{H_{A} \approx \frac{W_{1}}{{\tan\Theta_{B1}} + {\tan\Theta_{B2}}}}} & (5) \end{matrix}$

Defining Θ_(B4) as the angle of deviation of the focal point A from the axis of symmetry 101 as of the first part 101 with width W1,

$\begin{matrix} {{\tan\Theta_{B4}} \approx \frac{{\tan\Theta_{B1}} - {\tan\Theta_{B2}}}{2}} & (6) \end{matrix}$

In the same way, the first jet beam NJ1 and third jet beam NJ3 intersect at point B with the coordinates (W_(B), H_(B)), where:

$\begin{matrix} {{W_{B} \approx {{\tan{\Theta_{B3} \cdot H_{B}}} - W_{2}}},{H_{B} \approx \frac{W_{1} + W_{2}}{{\tan\Theta_{B1}} + {\tan\Theta_{B3}}}}} & (7) \end{matrix}$

Defining Θ_(B5) as the angle of deviation of the focal point B from the axis of a symmetry of the device 100 (i.e. the Z-axis in FIG. 1) with width W1+W2,

$\begin{matrix} {{\tan\Theta_{B5}} \approx \frac{{W_{2}\tan\Theta_{B1}} - {W_{1}\tan\Theta_{B3}}}{W_{1} + W_{2}}} & (8) \end{matrix}$

The second NJ2 and third NJ3 jet beams intersect only if n₃ is above a critical value, i.e. if n₃≥√{square root over (n₁n₂)}. In this case the coordinates of the point C are determined as:

$\begin{matrix} {{W_{C} \approx {{\tan\;{\Theta_{B3} \cdot H_{C}}} - W_{2}}},{H_{c} \approx \frac{W_{2}}{{\tan\Theta_{B3}} - {\tan\Theta_{B2}}}}} & (9) \end{matrix}$

In this case, defining Θ_(B6) as the angle of deviation of the focal point C from the axis of symmetry 102 as of the second part 102 with width W2,

$\begin{matrix} {{\tan\Theta_{B6}} \approx \frac{{\tan\Theta_{B2}} + {\tan\Theta_{B3}}}{2}} & (10) \end{matrix}$

The particular case where the three jet beams NJ1, NJ2 and NJ3 intersect at the same point for fixed refractive indexes of the two parts 101, 102 and of the host medium 103 can be obtained as the result of variation of the widths W1, W2 of the two parts 101, 102. In order to get an intersection of the three jet beams NJ1, NJ2 and NJ3 at one point, the ratio W₁/W₂ has to be equal to:

$\begin{matrix} {\frac{W_{1}}{W_{2}} \approx \frac{{\tan\Theta_{B1}} + {\tan\Theta_{B2}}}{{\tan\Theta_{B3}} - {\tan\Theta_{B2}}}} & (11) \end{matrix}$

In this case, all three jet beams NJ1, NJ2 and NJ3 contribute to the total generated focused electromagnetic beam radiated by the device 100. Thus, the intensity of the generated focused electromagnetic beam is maximal.

The dependencies of the deviation angles Θ_(B4-B6) on the refractive index n₃ for the fixed value n₂=1.8 and W₁=W₂ are presented in FIG. 2. For the chosen parameters, the critical value n_(3cr) of refractive index n₃ is given by, n_(3cr)=√{square root over (n₁n₂)}=1.34. In that case, the second NJ2 and third NJ3 jet beams intersect when n₃>n_(3cr). So, until n₃ reaches this critical value n_(3cr), the angle Θ_(B6) has no meaning.

5.2.2 Parametric Study

Based on the identified properties of the device 100 depicted in FIG. 1, the parameters of the device 100 (i.e. the refractive index values, and/or the widths and/or the heights of the first part 101 and of the second part 102) may be adapted in order to obtain a tilt of the generated focused electromagnetic beam (resulting from the combination of at least two beams among the first jet beam NJ1, the second jet beam NJ2 and the third jet beam NJ3) in respect of the direction of propagation of the incoming electromagnetic wave IEM radiated by the electromagnetic source 100 s.

Impact of the Respective Heights H1 and H2 of the First Part 101 and of the Second Part 102

The following numerical simulations have been performed assuming that H1=H2=H, W1=W2, and W=W1+W2 is greater than the equivalent wavelength in the host medium, λ, of the incoming electromagnetic wave IEM. However, as indicated in relation with FIG. 1, the technical effect of having the electromagnetic field lines of the outgoing focused electromagnetic beam presenting a curvature is achieved even if H1 and H2 are different from each other as long as |H2−H1|≤λ/4.

Under the present assumptions, it can be shown that the hot spots positions of the generated jet beams are almost independent from the height H of the device 100 for n₃<√{square root over (n₁n₂)} as illustrated e.g. in FIG. 3a for n₃=1.3.

For n₃>√{square root over (n₁n₂)} and starting from H1≈H_(A), a shift of the hot spot toward increasing X coordinates is observed. The power density distribution along the X-axis for n₃=1.6 is presented in FIG. 3b . For the parameters of the device 100 chosen for the simulation depicted in FIG. 3b , it appears that H_(A)≈772 nm.

FIGS. 4a to 4c display the power density distribution in the XZ-plane for three different heights H=H1=H2 of the device 100 assuming that n₃>√{square root over (n₁n₂)}. The power density distribution in question show that the curvature of the electromagnetic field lines of the outgoing focused electromagnetic beam is achieved by changing the height H of the device 100.

FIG. 4d presents the schematic distribution of the jet beams created by the device 100. It appears that the curvature of the electromagnetic field lines of the outgoing focused electromagnetic beam takes place if H1≥H_(A) and if the focal point A for the first jet beam Nil and the second jet beam NJ2 is close to the radiating surface 100 r of the device 100, or even within the device 100 (see FIG. 4d ). It can be seen, that in this case the curvature of the electromagnetic field lines of the outgoing focused electromagnetic beam is caused by an interplay of the second jet beam NJ2 and third jet beam NJ3, which are longer but less intensive than the first NJ1 jet beam. The angle of deviation of the generated focused electromagnetic beam is close to the angle Θ_(B6) of deviation of the focal point C from the axis of symmetry 102 as of the second part 102 with width W2 (dash-dot line 400 in FIG. 4d ).

FIGS. 5a and 5b show the dependencies of the hot spots positions of the jet beams generated by the device 100 (along the X and Z axis respectively) on the refractive index n₃ for three different heights H=H1=H2 of the device 100. More particularly, the light gray and the dash-dot lines indicate respectively the dependencies of the X and Z coordinates for points A and B on n₃. These two curves are obtained using equations (5) and (7). The power density of the hot spots as a function of n₃ is presented in FIG. 5 c.

The equivalent wavelength in the host medium 2 of the incoming electromagnetic wave influences the hot spots positions of the generated jet beams. FIGS. 6a and 6b show the dependency of the hot spots shift on A for different heights H=H1=H2. For n₃=1.3, the shift of the hot spots is almost independent on the wavelength λ. The influence of the dispersion rises with the refractive index n₃ (see FIG. 6b for n₃=1.6).

Thus, in some embodiments of the present disclosure, the device 100 is configured such as n₃≥√{square root over (n₁n₁)}, W₁=W₂ and H1≥H_(A), with H_(A) the Z coordinate of the intersection point of the first jet beam NJ1 and of the second jet beam NJ2 (i.e. the height, along the Z-axis and relative to the illumination face 100 i, of the intersection point of the first NJ1 and second NJ2 jet beams). This allows obtaining a tilt of the direction of the generated focused electromagnetic beam which is obtained from a combination of at least two beams among the first jet beam NJ1, the second jet beam NJ2 and the third jet beam NJ3 in respect of the direction of propagation of the incoming electromagnetic wave IEM.

Impact of the Respective Widths W1 and W2 of the First Part 101 and of the Second Part 102

This part of the description deals with the influence of the width W1 and W2 of the first part 101 and of the second part 102 of the device 100.

The influence of the width W2 of the second part 102, having the refractive index n₃, on the parameters of the generated jet beams is analyzed assuming a fixed width W1 of the first part 101 that has the refractive index n₂ (n₂>n₃>n₁).

More particularly, for n₃<√{square root over (n₁n₂)}, two jet beams emerge in the proximity of the radiating face 100 r of the device 100 when W₂>W₁ as illustrated in FIG. 7a . It appears that the angles of deviation of the first jet beam NJ1 (beam number 1 in FIG. 7a ) and of the second jet beam NJ2 (beam number 2 in FIG. 7a ) are equal to Θ_(B4) and Θ_(B5) (see FIG. 7b ). Thus, the position and the intensity of the hot spot (i.e. the point of highest intensity in the jet beam) of the second jet beam NJ2 depend on the width W₂.

The power density distribution along the X-axis at different widths W2 of the second part 102 of the device 100 is presented in FIG. 7c . It appears that the intensity and length of the second jet beam NJ2 rises with W₂. It also appears that increasing the total width W=W1+W2 of the device 100 increases the contribution of the Fresnel diffraction phenomenon into the total response of the system. Thus, the number of side jets of second type (or secondary lobes) deviated at an angle Θ_(B5) rises as W₂ increases.

Alternatively, when n₃>√{square root over (n₁n₂)}, two additional jet beams are obtained (e.g. as in the configuration of FIG. 8a ). The angle of deviation of the third jet beam NJ3 (beam number 3 in FIG. 8a ) associated with the third contact area 100 e 3 is equal to Θ_(B6) (see FIG. 8b ). The central jet beam (beam number 4 in FIG. 8a ) is directed along the Z-axis (normal to the radiating face 100 r of the device). The power density in the hot spots of the jet beams number 2-4 in FIG. 8a and the positions of the corresponding jet beams' hot spots versus the width W₂ of the second part 102 are presented in FIGS. 8b to 8e . As discussed above in the case where n₃<√{square root over (n₁n₂)}, the increase of W₂ increases the contribution of the Fresnel diffraction phenomenon into the total response of the system. Consequently, the number of side jets of second type (or secondary lobes) deviated at angles Θ_(B5) (left side when looking at the figures) and Θ_(B6) (right side when looking at the figures) increases as W₂ increases.

Thus, in some embodiments of the present disclosure, the device 100 is configured such that n₃<√{square root over (n₁n₂)}, and W2>W1, independently of the heights H1 and H2 of the first part 101 and of the second part 102 of the device 100 (as long as the condition |H2−H1|≤λ/4 is fulfilled). Such parameters allow obtaining a tilt of the direction of the generated focused electromagnetic beam (that results from a combination of at least two beams among the first jet beam NJ1, the second jet beam NJ2 and the third jet beam NJ3) in respect of the direction of propagation of the incoming electromagnetic wave IEM radiated by the electromagnetic source 100 s.

There is also an influence of the width W1 of the first part 101 having the refractive index n₂, on the parameters of the generated jet beams assuming a fixed width W2 of the second part 102 that has the refractive index n₃ (n₂>n₃>n₁).

In fact, a similar behavior can be observed as in the previous case discussed above where W2 was varying for a fixed value of W1. However, in the present case, the side jet beams (or secondary lobes) are deviated toward the angles Θ_(B4) (for the side jet beams on the left side when looking at the figures) and Θ_(B6) (for the side jet beams on the right side when looking at the figures). As mentioned above, in accordance with equation (11) the generated focused electromagnetic beam can be intensified based on some particular dimensions of the device 100. For the values of the parameters of the system as chosen in FIGS. 9a-d , the intersection of all the jet beams occurs when W₁=1960 nm. In FIG. 9d , the maximal intensity of the generated focused electromagnetic beam corresponds to W₁≈1600 nm. The maximal benefit of the intersection of all jet beams at the same point will be observed at W≈λ, where W=W1+W2.

Thus, in some embodiments of the present disclosure, the device 100 is configured such that n₃<√{square root over (n₁n₂)}, W2<W1, and H1≥H_(A) (still with |H2−H1|≤λ/4). Such parameters allow obtaining a tilt of the direction of the generated focused electromagnetic beam (that results from a combination of at least two beams among the first jet beam NJ1, the second jet beam NJ2 and the third jet beam NJ3) in respect of the direction of propagation of the incoming electromagnetic wave IEM.

With the sizes W₁>W₂, the deviation to the right (when looking at the figures) is observed for n₃<√{square root over (n₁n₂)} (e.g. as in the configuration of FIG. 102) and even at lower heights H1, H2 of the parts 101, 102 of the device 100 when n₃>√{square root over (n₁n₂)} as long as H1≈H_(A)−λ/2, (e.g. as in the configuration of FIG. 10b ). Several factors are responsible for these phenomena. First of all, increasing the width W₁ increases the distance H_(A). Thus, the main contribution to the total focused electromagnetic beam radiated by the device 100 comes from the longer jet beams NJ2 and NJ3 radiated at the angles Θ_(B2) and Θ_(B3).

Thus, in some embodiments of the present disclosure, the device 100 is configured such that n₃>√{square root over (n₁n₂)}, W2<W1, and H1 is targeted to be equal to H_(A)−λ/2 (still with |H2−H1|≤λ/4). Such parameters allow obtaining a tilt of the direction of the generated focused electromagnetic beam (that results from a combination of at least two beams among the first jet beam NJ1, the second jet beam NJ2 and the third jet beam NJ3) in respect of the direction of propagation of the incoming electromagnetic wave IEM.

Impact of Illumination Conditions

Examination of power density distributions obtained for different oblique angles of incidence, a, (i.e. the angle of incidence of the incoming electromagnetic wave IEM relative to a normal of the illumination face 100 i) shows that the tilt of the direction of the generated focused electromagnetic beam in respect of the normal to the radiating face 100 r is sensitive to the height of the device 100.

For example, when the device 100 is in contact with a dielectric host medium 103 such that n₁=1, n₂=1.8, n₃=1.6, W₁=1000 nm, W₂=700 nm, H1=H2=H=300 nm and α=10°, the tilt angle γ of deviation of the generated focused electromagnetic beam from the normal incidence is equal to 14.49° and remains constant for all wavelengths of incident waves in the theoretical hypothesis of non-dispersive materials. The slight dispersion in the case of α=10° is observed for H=550 nm. Moreover, the angle γ is approximately equal to 17.54°. The dispersion of the system rises with the height H. The comparison of FIG. 11a and FIG. 11b shows that for H=900 nm, the dispersion for normal illumination is lower than the dispersion for an oblique one. Moreover, for α=10°, the angle γ of deviation of the generated focused electromagnetic beam changes from ≈21° to ≈18.9° for 450 nm<λ<650 nm. For this height H=H1=H2 of the device 100, the additional deviation is approximately equal to the angle of incidence. A similar behavior of the generated focused electromagnetic beam takes place for higher angles of incidence α. But in this case, the dispersion of deviation is higher. In some embodiments, the angle of incidence of the incoming EM wave 100 i lays in the range of −20 degrees and +20 degrees. Preferably, such angle lays between −10 degrees and +10 degrees, more preferably between −5 degrees and +5 degrees.

One can note that the present simulations have been performed for H=H1=H2, however, as discussed above, the conclusion remains the same as long as |H2−H1|≤λ/4.

Impact of the Base Angle of the First Part 101 and of the Second Part 102

According to the embodiment of FIG. 12, a device 100′ with nonvertical edges (i.e. edges not parallel to the Z-axis) is considered whereas the radiating 100 r and illumination 100 i faces both remain orthogonal to the Z-axis. αj (with j equal to 1, 2 or 3) represent the base angles of the contact areas 100′e 1, 100′e 2 and 100′e 3. With αj different from 90°, the device 100′ may correspond to a double-material prismatic device in contact with a dielectric host medium 103, with a refractive index n1<n3<n2.

It can be shown that for the device 100′ with the base angles αj greater than 90°−Θ_(Bj), with j equal to 1, 2 or 3, the corresponding jet beam radiation angle can be determined using the approximate formula:

$\begin{matrix} {\Theta_{Bj} \approx \frac{90^{\circ} - \Theta_{TIRj}^{*}}{2}} & (12) \end{matrix}$

where the angles θ′_(TIRj) are the critical angles of refraction from the nonvertical edges, or contact areas 100′e 1, 100′e 2 and 100′e 3.

To get the approximate formula for θ′_(TIRj), the changing of the position of the contact areas compared to the configuration of the device 100 of FIG. 1 is considered. As a result, the corresponding jet beams radiation angle can be estimated as:

$\begin{matrix} {\Theta_{Bj} \approx {90^{\circ} - \frac{\Theta_{TIRj} + \alpha_{j}}{2}}} & (13) \end{matrix}$

These expressions can then be injected into the equations (4)-(11) for having a description of the behavior of the jet beams associated with the contact areas 100′e 1, 100′e 2 and 100′e 3 of the device 100′.

To consider the effect of the first part 101′ and of the second part 102′ with nonvertical contact areas 100′e 1, 100′e 2 and 100′e 3, simulation results are discussed below that show the influence of the height H1=H2=H of the first part 101′ and of the second part 102′ on the generated jet beams when the total width W′=W′1+W′2 is such that W′>λ with W′1=W′2. It is seen that the hot spot position is almost independent from the height H′ of the device 100′ that is in contact with the dielectric host medium 103 when n₃<√{square root over (n₁n₂)} (see FIG. 13a for n₃=1.3). For n₃>√{square root over (n₁n₂)} starting from the specific height H≈HA, a deviation of the hot spot to the right (when looking at the figure) is observed. The power density distribution along the X-axis for n₃=1.6 is presented in FIG. 13b . For the parameters of the device 100′ and of the dielectric host medium 103 chosen in those FIGS. 13a and 13b , H_(A)≈718.96 nm.

FIGS. 14a to 14c display the power density distribution in the XZ-plane for 3 different heights H′=H′1=H′2 of the device 100′ with n₃>√{square root over (n₁n₂)}. It can be shown that for the device 100′ with nonvertical contact areas 100′e 1, 100′e 2 and 100′e 3, the deviation of the generated focused electromagnetic beam can also be achieved by changing the height H′=H′1=H′2 of the device 100′.

More particularly, FIGS. 152 to 15 d represent the height H_(A) and the deviation angle Θ_(B6) as obtained based on equations (5) and (10). FIG. 15a shows that varying the angle α₁ at fixed angles α₂ and α₃ affects only the specific height H_(A) without changing the angle Θ_(B6) (for the parameters chosen for the simulation plotted in FIG. 15 a, Θ _(B6)=22°). Conversely, decreasing or increasing the base angle α₂ leads to the changing of both parameters H_(A) and Θ_(B6) as can be seen in FIGS. 15b and 15c . For the minimal angle α₂ a maximal deviation of the corresponding jet beam at minimal height H′=H′1=H′2 of the device 100′ is obtained.

As can be seen in FIG. 15d , changing the base angle α₃ affects the specific height H_(A) (for the parameters chosen for the simulation plotted in FIG. 15d H_(A)=718.96 nm). The maximal deviation is observed for the minimal angle α₃.

FIG. 16 gives the power density distribution along the X-axis in the cut-plane Z₀=1000 nm for four different values of the angle α₃, with λ=550 nm, with α₁=85°, α₂=90° and with the device 100′ having H′=H′1=H′2 greater than H_(A).

5.3 Optical Manipulation Apparatus

FIGS. 17a to 19d relate to subwavelength optical manipulation of micro or nanoparticles by the curved jet beams generated by a device (e.g. device 100 or 100′) according to the disclosure.

More particularly, the optical forces acting on arbitrary micro or nanoparticles can be obtained by approximating the particle by an electric dipole.

This method applies for Rayleigh particles (particles much smaller than the incident wavelength) as detailed for instance in the article by P. Chaumet and M. Nieto-Vesperinas: “Time-averaged total force on a dipolar sphere in an electromagnetic field,” Opt. Lett. 25, 1065-1067 (2000).

Using the dipole approximation for the subwavelength radius of the sphere, the force can be obtained as:

$\begin{matrix} {F = {{\frac{1}{2}{{Re}(\chi)}{\sum\limits_{i}{{Re}\left( {E_{i} \cdot {\nabla E_{i}^{*}}} \right)}}} + {\frac{1}{2}{{Im}(\chi)}{\sum\limits_{i}{{{Im}\left( {E_{i} \cdot {\nabla\; E_{i}^{*}}} \right)}.}}}}} & (14) \end{matrix}$

where

$\chi = {\pi n_{m}^{2}a^{3}\frac{m^{2} - 1}{m^{2} + 2}}$

is the particle's complex polarizability,

${m = \frac{n_{p}}{n_{m}}},n_{p}$

is the refractive index of the material of the particle, n_(m) is the refractive index of the medium outside the particle and a is the radius of the particle.

The forces produced by the deviated jet beams in the presence or not of obstacles 1800 have been simulated using Comsol software and based on the dipolar approximation for gold particles with a radius of 0.03 μm and a dielectric permittivity ε=−11.208+1.31184i. Further, those forces have been simulated on one hand for a single material device 1710 having a refractive index n, and for the dual-material device 100 of FIG. 1.

In FIGS. 17a and 17b , the device is illuminated by a TM (for “Transverse magnetic”) polarized incoming electromagnetic wave IEM with λ=550 nm and α=0° (i.e. the incoming electromagnetic wave IEM has a direction of propagation orthogonal to the illumination face 100 i of the device 100). The single material device 1710 is made of the same material as the first part 101 of the device 100, i.e. n=n₁=n₂. Further, the single material device 1710 has the same width W as the device 100, with W=W1+W2=2*600 nm (W₁=W₂), and the same height H=H1=H2=900 nm. The streamlines and arrows show the trajectories and directions of the optical forces obtained using the dipolar approximation. It is possible to see that in case of double material device 100, it is possible to transport the particles along the curved trajectory.

Referring to FIGS. 18a to 19d , an obstacle 1800 is placed along the jet beams paths. The length of the obstacle 1800 in X-direction coincides with the width W1 of the first part 101 of the device 100. However, the obstacle 1800 is shifted between the axis of symmetry of the obstacle 1800 and the axis of symmetry the first part 101 by an offset equal to W₁/4. The same configuration is used for the single material device 1710. The simulations correspond to an obstacle 1800 made of glass with a refractive index 1.8 and a thickness equal to λ/8 placed at the distance 5λ/8 in the case of FIGS. 18a to 18d , and made of metal (which is a perfect electric conductor) with the same thickness λ/8 placed at the distance 3λ/8 in the case of FIGS. 19a to 19d . The presence of the obstacle 1800 leads to the disruption of the jet beams and to valuable changing in the trajectory. By increasing the refractive index of the single material device 1710, the curved trajectory is obtained but the forces pushing the particle away from the device 100 are very small. Only the double-material device 100 (here with the height H=H1=H2=900 nm) can generate the deviated jet beam. In that case, the particles can move along the curved trajectory around the obstacle. This effect was obtained for both type of obstacles 1800 (i.e. glass and metal). Moreover, in case of a conducting obstacle 1800, trapping locations near the obstacle can be obtained (white circle referred 1900 in FIG. 19c ).

Thus, such double-material device 100, 100′ according to the disclosure (in any of its embodiments discussed above) can be used as an elementary part of an optical manipulation apparatus (e.g. an optical tweezer) 1700 for trapping or moving micro or nanoparticles in the dielectric host medium 103. For instance, the equivalent wavelength in the host medium, λ, of the incoming electromagnetic wave IEM belongs to the visible light spectrum (e.g. the wavelength in the host medium 2 lays between 400 nm and 700 nm, or equivalently the frequency of the incoming IEM wave lays between 430 THz to 790 THz). Thus, devices with nano-scale dimensions can be obtained for forming visible light wave patterns. In other embodiments, the frequency of the incoming IEM wave belongs to the group comprising:

radio waves, i.e. between 30 Hz to 300 GHz;

microwaves, i.e. 1 GHz to 100 GHz;

terahertz radiations, i.e. 100 GHz to 30 THz;

infrared, i.e. 300 GHz to 430 THz; and

ultraviolet, i.e. 790 THz to 30 PHz.

In those embodiments, the size of the device 100, 100′ is thus scaled according to the wavelength of the incoming IEM wave in order to achieve the tilt of the generated focused electromagnetic beam.

In some embodiments, an optical manipulation system comprises the optical manipulation apparatus 1700 and the electromagnetic source 100 s for radiating the incoming electromagnetic wave IEM that illuminates the illumination face 100 i of the device 100, 100′ (in any of its embodiments discussed above). 

1. A device configured to radiate a focused electromagnetic beam in a dielectric host medium having a first refractive index n₁, when an incoming electromagnetic wave illuminates a first face of the device, named illumination face, wherein the device comprises: a first material having a second refractive index n₂ and having a first width W₁ along a direction of extension of the first face, named X-axis; and a second material in contact with the first material and having a third refractive index n₃, having a second width W₂ along the X-axis, with n1<n3<n2, and with W₁+W₂ being greater than an equivalent wavelength λ in the host medium of the incoming electromagnetic wave, the first and second materials extending along a direction orthogonal to the illumination face, named Z-axis, from the illumination face up to a radiating face of each material, opposite to the illumination face, the first and second materials having respectively a first height H1 and a second height H2 along the Z-axis, where |H2−H1|≤λ/4, the device being configured to have, when the device is in contact with the dielectric host medium and when the incoming electromagnetic wave illuminates the illumination face: a first contact area between the dielectric host medium and the first material, the first contact area radiating a first jet beam in a near field zone; a second contact area between the first material and the second material, the second contact area radiating a second jet beam in the near field zone; and a third contact area between the material and the dielectric host medium, the third contact area radiating a third jet beam in the near field zone, wherein the focused electromagnetic beam results from a combination of at least two beams among the first, second, and third jet beams, the device being configured for having a direction of propagation of the focused electromagnetic beam being tilted in respect of a direction of propagation of the incoming electromagnetic wave.
 2. The device according to claim 1, wherein the direction of propagation of the focused electromagnetic beam is tilted in respect of a direction of propagation of the incoming electromagnetic wave as a function of at least part of: the first n₁, second n₂ and third n₃ refractive indexes; the first W₁ and second W₂ widths; and the first H1 and second H2 heights.
 3. The device according to claim 1, wherein the focused electromagnetic beam results from a combination of the first, second and third jet beams.
 4. The device according to claim 1, wherein n₃≥√{square root over (n₁n₂)}, wherein W₁=W₂, and wherein H1≥H_(A), with H_(A) a height, along the Z-axis and relative to the illumination face, of the intersection point of the first and second jet beams.
 5. The device according to claim 1, wherein n₃<√{square root over (n₁ n₂)} and wherein W₂>W₁.
 6. The device according to claim 1, wherein n₃<√{square root over (n₁n₂)}, wherein W₂<W₁, and wherein H1≥H_(A), with H_(A) the height, along the Z-axis and relative to the illumination face, of an intersection point of the first and second jet beams.
 7. The device according to claim 1, wherein n₃>√{square root over (n₁n₂)}, wherein W₂<W₁, and wherein H1 is targeted to be equal to H_(A)−λ/2, with H_(A) the height, along the Z-axis and relative to the illumination face, of an intersection point of the first and second jet beams.
 8. The device according to claim 4, wherein the height H_(A) fulfils ${H_{A} = \frac{W_{1}}{{\tan\;\Theta_{B1}} + {\tan\;\Theta_{B2}}}},$ Θ_(B1) and Θ_(B2) being respectively radiation tilt angles of the first and second jet beams in respect of the direction of propagation of the incoming electromagnetic wave.
 9. The device according to claim 8, wherein Θ_(B1) is equal to ${90^{\circ}} - \frac{\Theta_{{TIR}\; 1} + \alpha_{1}}{2}$ and Θ_(B2) is equal to ${{90^{\circ}} - \frac{\Theta_{{TIR}\; 2} + \alpha_{2}}{2}},$ where angles α₁ and α₂ are respectively the base angles of the first and second contact areas relative to the X-axis, and where Θ_(TIR1) and Θ_(TIR2) are limit angles of refraction associated with, respectively, the first and third contact areas.
 10. The device according to claim 9, wherein $\Theta_{{TIR}\; 1} = {{\sin^{- 1}\left( \frac{n_{1}}{n_{2}} \right)}\mspace{20mu}{and}}$ $\Theta_{{TIR}\; 2} = {{\sin^{- 1}\left( \frac{n_{3}}{n_{2}} \right)}.}$
 11. The device according to claim 1, wherein the equivalent wavelength in the host medium, λ, of the incoming electromagnetic wave belongs to the visible light spectrum.
 12. The device according to claim 1, wherein at least one of the first and second materials belongs to the group comprising: glass; plastic; a polymer material; oxides; and nitrides.
 13. An optical manipulation system comprising: a device configured to radiate a focused electromagnetic beam in a dielectric host medium having a first refractive index n₁, when an incoming electromagnetic wave illuminates a first face of the device, named illumination face, wherein the device comprises: a first material having a second refractive index n₂ and having a first width W₁ along a direction of extension of the first face, named X-axis; and a second material in contact with the first material and having a third refractive index n₃, having a second width W₂ along the X-axis, with n1<n3<n2, and with W₁+W₂ being greater than an equivalent wavelength λ in the host medium of the incoming electromagnetic wave, the first and second materials extending along a direction orthogonal to the illumination face, named Z-axis, from the illumination face up to a radiating face of each material, opposite to the illumination face, the first and second materials having respectively a first height H1 and a second height H2 along the Z-axis, where |H2−H1|≤λ/4, the device being configured to have, when the device is in contact with the dielectric host medium and when the incoming electromagnetic wave illuminates the illumination face: a first contact area between the dielectric host medium and the first material, the first contact area radiating a first jet beam in a near field zone; a second contact area between the first material and the second material, the second contact area radiating a second let beam in the near field zone; and a third contact area between the material and the dielectric host medium, the third contact area radiating a third let beam in the near field zone, wherein the focused electromagnetic beam results from a combination of at least two beams among the first, second, and third let beams, a direction of propagation of the focused electromagnetic beam being tilted in respect of a direction of propagation of the incoming electromagnetic wave; and an electromagnetic source configured to generate the incoming electromagnetic wave.
 14. The device according to claim 1, wherein the device is configured to trap or move microparticles or nanoparticles in the dielectric host medium.
 15. A method comprising: trapping or moving micro or nanoparticles in a dielectric host medium, wherein trapping or moving the micro or nanoparticles in the dielectric host medium uses a device configured to radiate a focused electromagnetic beam in a dielectric host medium having a first refractive index n₁, when an incoming electromagnetic wave illuminates a first face of the device, named illumination face, wherein the device comprises: a first material having a second refractive index n₂ and having a first width W₁ along a direction of extension of the first face, named X-axis; and a second material in contact with the first material and having a third refractive index n₃, having a second width W₂ along the X-axis, with n1<n3<n2, and with W₁+W₂ being greater than an equivalent wavelength in the host medium of the incoming electromagnetic wave, the first and second materials extending along a direction orthogonal to the illumination face, named Z-axis, from the illumination face up to a radiating face of each material, opposite to the illumination face, the first and second materials having respectively a first height H1 and a second height H2 along the Z-axis, where |H2−H1|≤λ/4, the device being configured to have, when the device is in contact with the dielectric host medium and when the incoming electromagnetic wave illuminates the illumination face: a first contact area between the dielectric host medium and the first material, the first contact area radiating a first jet beam in a near field zone; a second contact area between the first material and the second material, the second contact area radiating a second jet beam in the near field zone; and a third contact area between the material and the dielectric host medium, the third contact area radiating a third jet beam in the near field zone, wherein the focused electromagnetic beam results from a combination of at least two beams among the first, second, and third jet beams, a direction of propagation of the focused electromagnetic beam being tilted in respect of a direction of propagation of the incoming electromagnetic wave. 